If you're mingy, you're a penny pincher — in other words, you are not generous about spending your money or sharing it with others. A mingy friend won't offer to buy you lunch, or even a jawbreaker.
The adjective mingy is an informal way to describe people who don't like to part with their money, people you might describe as miserly. A mingy restaurant patron is likely to leave a very small tip for his waiter, and it could take years before you get a raise if you work for a mingy boss. Mingy can be used as a variation on stingy, and in fact it probably came from a combination of mean and stingy.